Method for making fine wire grids



June 19, 1956 v, NEHER 2,750,655

METHOD FOR MAKING FINE WIRE GRIDS Original Filed Dec. 11, 1945 FIG. I

r\\ [Hill llll 7 FIG. 3

30 ..|1 3 s l! lblr "lluilllllni H I 51% FI6.4 FIG.5 y

Arno y United. States Patent METHOD FOR MAKING FINE WIRE GRIDS Henry V. Neher, Pasadena, Calif., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Original application December 11, 1945, Serial No. 634,295. Divided and this application March 23, 1950, Serial No. 151,460

4 Claims. (Cl. 29-25.18)

This invention relates to ultra-high frequency oscillators and is a division of my copending application Serial No. 634,295, filed December 11, 1945, for Controllable Oscillator Tube.

This invention relates to an electron discharge device and more particularly, to a low-voltage velocity-modulated reflex type of oscillator incorporating grids. The output frequency of this oscillator, which is in and above the Super High Frequency (SI-IF) band of the spectrum as defined and standardized by the Federal Communications Commission, is adjustable by means of an electrical circuit.

Conventional velocity-modulated reflex oscillators operating in and above the high frequency end of the SHF band employ an electron gun, the emitted electron stream of which is caused to pass through small apertures placed along the axis of radial symmetry of the resonant cavity. It has been determined that for the condition of maximum power output the transit time of the electrons through the region of high field intensity surrounding the apertures should be approximately equal to one-half the natural period of the resonant cavity. Due to the tendency for the field to diverge if the size of the apertures is increased, either the apertures must be made small or the electron velocity must be increased to keep the transit angle approximately equal to 180 electrical degrees. Because of these factors, such velocity-modulated reflex oscillators require electrode operating potentials of 1800 to 2100 volts and have apertures which are not so small as to be difficult to manufacture uniformly.

Adjustment of the operating frequency over wide limits is usually accomplished by mechanically changing the physical spacing between the apertures in the resonant cavity of such an oscillator, while adjustment over narrow limits may be accomplished by variation of the negative potential difference of the reflector with respect to the cathode. The degree of frequency variation by the latter means is a direct function of the electrical losses involved in the frequency-determining impedance associated with the oscillator. Thus the operating frequency of an oscillator having low electrical losses in its resonant circuit cannot be varied over as Wide a range by changing the reflector voltage as can that of an oscillator having high losses.

Coupling of the integral resonant circuit to a transmission line is usually accomplished by means of a singleturn inductor inserted into the electromagnetic field produced by the resonant circuit, the degree of coupling being a function of the position of the inductor relative to the field. The small physical dimensions of the coupling inductor and the resonant circuit require the maintenance of extremely small tolerances in assembly if oscillators having reasonably uniform characteristics are to be obtained.

Disadvantages inherent in conventional oscillators of the type described are the high operating potentials required, the lack of convenient electrical means for re Patented June 19, 1956 'ice motely changing the operating frequency over wide ranges, and the difliculty in obtaining an easily reproducible means for coupling the generated power to an external circuit.

It is an object of this invention to provide a velocitymodulated reflex type of oscillator capable of operating at super high frequencies and above which uses fine wire grids and does not require the use of excessively high operating potentials.

This and other objects will be more apparent upon consideration of the following description together with the accompanying drawings, in which:

Fig. l is a sectional view of an oscillator having grids embodying the invention;

Fig. 2 is a plan view of the second grid mounted on a truncated cone;

Fig. 3 is a sectional view along line III-III of Fig. 2;

Fig. 4 is a plan view of the third grid on its mounting ring;

Fig. 5 is a sectional view along line V-V of Fig. 4.

Specifically, with reference to the embodiment of this invention shown in Fig. l, the oscillator comprises an electron gun consisting of a heater 10, a cathode 11, and a focusing electrode 12 designed to operate at the cathode potential. The electron gun assembly is mounted on a mica insulating support 9. The first grid 13 is so positioned in the electron stream that it produces an additional focusing effect and helps to increase the uniformity of electron emission over the surface of the cathode.

The electron stream is caused to flow through apertures along the axis of radial symmetry of the resonant cavity 15. A plurality of substantially parallel wires forming a grid is placed across each aperture so that the high-intensity field is confined largely within the space between the grids. The second grid 14 is mounted on the end of a truncated cone 34, which forms the bottom of the resonant cavity, while the third grid 30 is mounted on a flat ring. The procedure for making the second grid 14 is as follows:

A one-half inch square sheet of 0.010 inch molybdenum with a 0.250 inch hole in the center is wound with 0.0006 inch diameter tungsten Wire with a spacing of 0.0037 inch. A truncated cone 34, as shown in Figs. 2 and 3, is formed from 0.005 inch thick low carbon content steel on which a 0.001 inch to 0.002 inch thick coating of copper is applied by electro-deposition. The copper-plated cone is placed on the grid winding in an inverted position, with the tip of the cone resting on the tungsten grid wires and with the one millimeter hole in the tip of the cone coaxial with the 0.250 inch hole in the molybdenum sheet. These parts are placed in an oven in this position and heated together in a hydrogen atmosphere until the copper begins to flow, imbedding the tungsten wires in the copper around the edges of the one millimeter hole in the tip of the cone. Heating is stopped when the copper starts to flow to prevent surface tension from pulling the copper away from the tip of the cone. The heated parts are allowed to cool until the copper solidifies and the cone is then separated from the molybdenum plate, breaking off the grid wires outside the periphery of the cone tip. The excess wires are then either broken off close to the cone tip with fine tweezers or removed with a fine abrasive stone. The grid wires are bowed outward approximately 0.002 inch by inserting into the cone a rounded tool having a tip radius of 0.100 inch.

It is desirable that the cone be fabricated from a steel having a low carbon content, such as Swedish iron, since the tungsten wires carbonize during heating and become extremely brittle when higher carbon content steel is used.

The third grid 30, shown in Figs. 4 and 5, is made as follows:

A sheet of molybdenum 0.005 inch thick is pierced with a number of 0.046 inch diameter holes and is then given a 0.0005 inch coating of gold by electro-deposition. Two such sheets of gold-plated molybdenum are then clamped together with a third sheet, which has been coated with a ceramic or other material to which gold will not adhere, placed between them. The three sheets, forming a laminated unit, are then wound with tungsten wire having substantially the same diameter and spacing as that used in forming the second grid and are placed in an oven and heated in a hydrogen atmosphere until the gold fuses, causing the tungsten wires to be gold-soldered to the molybdenum sheets. The laminations are separated with a razor blade and disks concentric with the 0.046 inch diameter holes are cut with a blanking die. In this manner, a large number of grids with mounting rings can be fabricated with but one winding and heating operation. The grids 30 so formed are bowed by means of a rounded tool to conform with the profile of the second grid .14. Distortion of the grid wires in this manner insures that when they expand due to heating, the wires of both the first and second grids will move in the same predetermined direction and by the same amount, thus maintaining a constant separation between the grids. The performance of the oscillator depends critically upon the position and shape of the reflector 29. The tilt of the axis of the reflector must be maintained accurately to within one degree to prevent the phase variation of the reflected electrons from exceeding 90 electrical degrees. This precise alignment is accomplished by enclosing the reflector in a steel cylinder 32 and supporting it coaxially with a ceramic tube 28 which provides electrical insulation. The steel alignment cylinder is given a coating of silver and copper in approximately eutectic proportions by electrodeposition.

A ring of nickel wire 26 is spot-welded to the reflector conductor which passes through the ceramic tube 28 to fix the reflector 29 in the ceramic tube. The proper position of the reflector 29 with respect to the third grid 30 is maintained by means of ceramic cement 27.

A 0.0005 inch thick circular diaphragm 33 of a copper-plated alloy having a low coeflicient of thermal expansion, such as Kovar, and having concentric corrugations to provide axial flexibility, forms the top of the resonant cavity 15. The mounting ring containing the third grid 30, the silvercopper-plated reflector alignment cylinder 32 and the copper-plated Kovar diaphragm 33 are placed in a jig which holds the parts coaxial and spotwelded together. The assembly is then placed in an oven. and heated until the silver and copper plating, forming a solder, flows into the cracks and forms a fillet between the diaphragm and the grid ring.

The resonant cavity 15 is formed by a 0.230 inch diameter hole in a block 16 of copper-plated steel 0.040 inch thick, closed on one end by the cone 34 holding the second grid 1d and on the other by the diaphragm 33 and the third grid 30. One side of the cavity is cut away, thus forming an impedance matching transformer 35 between the resonant cavity, and the tapered waveguide transmission line 37. The dimensions of the impedance matching transformer 35, which is 0.020 inch long, 0.040 inch high and 0.095 inch wide, are so selected that a satisfactory compromise is effected between degree of coupling and the variation of impedance as a function of frequency.

Two sheets of copper-plated steel, each bent to form a tapered U-shaped channel are fastened to the flat surfaces of the cavity block. One such sheet is held between the cavity block 16 and the diaphragm 33 holding the third grid 30, while the other is mounted between the cavity block and the cone 34 holding the second grid 14. The sheets are spot-welded and silver-soldered together so as to form a waveguide transmission line 37 in the form ofa tube having a rectangular cross section tapering from inside dimensions of 0.040 inch by 0.410 inch to 0.170 inch by 0.410 inch. Cooling fins 36, formed from 0.005 inch thick copper sheet, are fastened to the tapered waveguide transmission line as shown in Fig. 1 to aid in dissipating the heat produced in the oscillator. A choke 41 having a circular slot electrical degrees deep and with the bottom of the slot placed 180 electrical degrees from the center of the wide edge of the waveguide is fastened to the large end of the tapered waveguide transmission line 37.

The choke is electrically insulated from and is held in position coaxial with a sheet steel envelope 44, which is preferably evacuated and encloses the complete apparatus shown in Fig. 1, by means of a mica ring 43, placed between the envelope and a skirt 38 fastened to the choke. Power is extracted from the evacuated envelope through a 0.250 inch diameter window 39 of low-loss glass having a definite thickness and being sealed into a cup 40 of metal having the same thermal coefficient as the low-loss glass, such as Kovar. The proper thickness of the glass window is a function of the dielectric constant of the glass at the operating frequency and may be so selected that the overall reflection coefficient of the glass window in combination with the choke is very low, resulting in a voltage standing-wave ratio of approximately 1.1 to 1.

A metal collar 42 is fastened to the top structure of the envelope to provide means for fastening the tube to the external transmission line fittings.

Tuning of the resonant cavity 15 is accomplished by varying the spacing between the second grid 14 and the third grid 30. A V-shaped tuning strut 18, consisting of two Nichrome strips 0.005 inch thick and 0.110 inch wide with the central portion stiffened by an embossed ridge, is fastened to the cavity block by means of a yoke 17. The vertex of the strut is fastened to the reflector alignment tube 32. The upper member of the strut 18 forms the anode of a triode electron tube 20 having a grid 23 and an indirectly heated cathode 22 held in position by mica supports 21. A decrease in the negative potential difference between the grid 23 and the cathode 22 causes an increase in space current. The resulting increase of electron bombardment of the anode causes it to heat and expand. The differential expansion thus produced between the two members of the strut causes the reflector alignment cylinder 32 to move toward the cathode 11. The spacing between the second and third grids, 14 and 30 respectively therefore decreases, causing a decrease in the frequency of the oscillations. A 0.003 inch thick by 0.100 inch wide copper strip 31 is fastened between the reflector alignment cylinder 32 and the cavity block 16 to conduct the heat away from the ends of the struts 18 and to conduct away the heat caused by intercepted current flowing in the third grid 30.

The oscillator herein described operates with a potential difference of 300 volts between the cathode and the resonant cavity structure and with a reflector potential of less than volts negative with respect to the cathode. The oscillator is tunable over a 6 per cent band of frequencies by means of any suitable electrical circuit controlling the potential difference between the grid and cathode of the triode 20, the tuning being accomplished in approximately 1.2 seconds.

Since certain changes may be made in the above described article and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense, and particularly, that dimensions of parts herein described be regarded as illustrative only and therefore. that the invention is to be limited only by the prior art and the spirit of the appended claims.

What is claimed is:

1. A method of making a grid wherein a sheet of metal is formed into a truncated cone having a hole at its tip,

said cone is coated with a relatively fusible metal, said cone is placed tip down upon a grid of wires tightly wound upon a frame, said cone, grid and frame are heated together in a chemically inert atmosphere until said relatively fusible metal begins to flow around said wires, heating is stopped before surface tension pulls said fusible metal back from the periphery of said hole, said cone, grid and frame are permitted to cool until said fusible metal is solidified, and-said frame and grid wires are removed from the outside of said hole leaving the remaining portions of said wires electrically connected and mechanically secured across said hole.

2. A method of making a fine wire grid wherein a sheet of steel is formed into a truncated cone having a hole at its tip, coating said cone with a layer of copper by electrodeposition, placing said cone tip down upon a frame having a grid of Wires wound thereon, heating said cone, grid and frame in hydrogen atmosphere to the fusion point of copper, cooling said cone, grid and frame to bond said grid wires electrically and mechanically to said cone, and separating said frame and excess wire from said cone.

3. A method of making a fine wire grid wherein a sheet of steel is formed into a truncated cone having a hole at its tip, coating said cone with a layer of copper by electrodeposition, placing said cone tip down upon a frame having a grid of wires wound thereon, heating said cone, grid and frame in hydrogen atmosphere to the fusion point of copper, cooling said cone, grid and frame to bond said grid wires electrically and mechanically to said cone, and separating said frame and excess wire from said cone, and mechanically deforming said grid wires to a predetermined contour.

4. The method of making a fine wire grid comprising the steps of winding tungsten wire on a molybdenum sheet having a hole therein to form a first grid, electrodepositing a copper coating on a truncated cone of low carbon content steel, placing said cone with the tip resting on said grid in a hydrogen atmosphere in an oven, heating until the copper begins to flow whereby to embed the tungsten wire in the copper to form a second grid, separating the cone with the embedded wires from the molybdenum sheet, removing the excess Wires, and deforming said second grid to a predetermined contour.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF MAKING A GRID WHEREIN A SHEET OF METAL IS FORMED INTO A TRUNCATED CONE HAVING A HOLE AT ITS TIP, SAID CONE IS COATED WITH A RELATIVELY FUSIBLE METAL, SAID CONE IS PLACED TIP DOWN UPON A GRID OF WIRES TIGHTLY WOUND UPON A FRAME, SAID CONE, GRID AND FRAME ARE HEATED TOGETHER IN A CHEMICALLY INERT ATMOSPHER UNTIL SAID RELATIVELY FUSIBLE METAL BEGINS TO FLOW AROUND SAID WIRES, HEATING IS STOPPED BEFORE SURFACE TENSION PULLS SAID FUSIBLE METAL BACK FROMT HE PERIPHY OF SAID HOLE, SAID CONE, GRID AND FRAME ARE PERMITTED TO COOL UNTIL SAID FUSIBLE METAL IS SOLIDIFIED, AND SAID FRAME AND GRIND WIRES ARE REMOVED FROM THE OUTSIDE OF SAID HOLE LEAVING THE REMAINING PORTIONS OF SAID WIRES ELECTRICALLY CONNECTED AND MECHANICALLY SECURED ACROSS SAID HOLE. 